In recent years, various kinds of electronics and information technology products such as portable telephones and personal computers have rapidly permeated into everyday life. On the other hand, to solve environmental issues and energy issues, electronics products are also expected to have high energy efficiencies. The center of such electronics products is formed by semiconductor integrated circuits having logical devices and storage devices integrated on semiconductor substrates, and the improvements of the performance, functions, and integrations achieved by miniaturization of those devices have supported the growth of electronics industry for decades. However, in view of the recent progress in miniaturization affected by the rapid increase in production costs and of requests for lower power consumptions, making a new breakthrough is critical.
As a technology to make a breakthrough, “nonvolatility” is described. When the power supply to a SRAM (Static Random Access Memory) mounted as a storage device on a semiconductor integrated circuit is switched off, stored information is lost. Therefore, current needs to be flowed to maintain data. On the other hand, nonvolatility means a function not to lose information even when the power supply is switched off. If a SRAM can be equipped with a nonvolatile function, the power supply could be switched off after information storage until the next information reading. Accordingly, power consumptions would be dramatically lowered.
To lower power consumptions, there are large expectations for nonvolatile memory development, and such development is accelerated around the world. Examples of nonvolatile memories include MRAM (Magnetic Random Access Memory), FeRAM (Ferroelectric Random Access Memory), PRAM (Phase change Random Access Memory), and ReRAM (Resistive Random Access Memory). Of those nonvolatile memories, only MRAM can be rewritten an infinite number of times, and characteristically has high writing and reading speeds. Accordingly, MRAM has a potential to realize nonvolatile working memory.
Among MRAMs, attention is drawn to MTJ (Magnetic Tunnel Junction) devices utilizing the spin-injection magnetization switching method and perpendicular magnetization film, because the write current of such MTJ devices is much lower than that of existing MRAMs. However, where spin-injection MRAM that has been developed as an alternative to nonvolatile SRAM and uses perpendicular magnetization film is considered, the writing speed poses a grave problem. According to the spin-injection magnetization switching method, when the writing time is 1 to 10 nanoseconds or shorter, the writing speed exceeds the magnetization switching speed, and accordingly, the write current rapidly increases. The increase in the write current poses grave problems, causing breaking of the tunnel barrier layer of each MTJ device and hindering high-speed operations.